Dual metal stud bumping for flip chip applications

ABSTRACT

A method for forming a stud bumped semiconductor die is disclosed. The method includes forming a ball at the tip of a coated wire passing through a hole in a capillary, where the coated wire has a core and an oxidation-resistant coating. The formed ball is pressed to the conductive region on the semiconductor die. The coated wire is cut, thereby leaving a conductive stud bump on the conductive region, where the conductive stud bump includes an inner conductive portion and an outer oxidation-resistant layer.

BACKGROUND OF THE INVENTION

There are many different levels of packages and interconnections inelectronic packages. In a typical first level packaging process, asilicon die is joined to a ceramic substrate carrier. In a typicalsecond level packaging process, the ceramic substrate carrier with thedie is mounted on an organic board.

In one conventional method for forming a first level package, apassivation layer is formed on a semiconductor die (which may be in asemiconductor wafer). The passivation layer includes apertures thatexpose conductive regions on the semiconductor die. Titanium and copperlayers are sputtered on the upper surface of the conductive regions andthe passivation layer. A layer of photoresist is then patterned on thesemiconductor die so that the apertures in the patterned photoresistlayer are over the conductive regions. Solder is electroplated in theapertures in the photoresist layer until the apertures are filled withsolder. The photoresist is stripped and the portions of the titanium andcopper layers around the solder deposits are removed. Then, the solderdeposits are subjected to a full reflow process. The full reflow processcauses the solder deposits to form solder balls. After forming thesolder balls, the semiconductor die is bonded face-down to a carrier.The solder balls on the semiconductor die contact conductive regions onthe chip carrier. Non-soluble barriers are disposed around theconductive regions and constrain the solder balls. The solder ballsbetween the conductive regions on the carrier and the semiconductor diemelt and wet the conductive regions on the carrier. Surface tensionprevents the melting solder from completely collapsing and holds thesemiconductor die suspended above the carrier.

During the reflow step, the deposited solder substantially deforms intosolder balls. Because of the deformation, the heights of the resultingsolder balls on the semiconductor die can be uneven. If the heights ofthe solder balls are uneven, the solder balls may not all contact theconductive regions of the carrier simultaneously when the semiconductordie is mounted to the chip carrier. If this happens, the strength of theformed solder joints may be weak thus potentially decreasing thereliability of the formed package. Moreover, during the reflow process,the deposited solder is exposed to high temperatures for extendedperiods of time. Excessively heating the deposited solder can promoteexcessive intermetallic growth in the solder deposits. Intermetallics inthe solder joints make the solder joints brittle and reduce the fatigueresistance of the solder joints. Lastly, performing a full reflowprocess takes time and energy and thus adds to the cost of the diepackage that is finally produced. If possible, it would be desirable toreduce the time and energy associated with the full reflow process.

One approach to solving the above problems is to use a “stud bumping”technique to form a copper stud on the conductive regions, instead ofsolder. The copper stud is formed using a wire bonding technique wherean end of a wire forms a ball, which is compressed to a conductiveregion of a semiconductor die. The wire is then cut leaving thecompressed ball, which is in the form of a copper stud. Thesemiconductor die is then flipped over and is then mounted to a carrierof a circuit board having conductive lands with Pb-Sb-Sn solder.

While the described stud bumping approach is feasible, there are someproblems to be addressed First, in the above-described approach, a thickintermetallic compound layer can form between the copper stud and thePb-Sb-Sn solder. The thick intermetallic compound layer can increase the“on resistance” of the die package. Second, voids can form between thecopper in the copper stud and the thick intermetallic compound layer. Asshown in FIG. 1, for example, after testing for 1000 hours at 150° C., agap (or void) is shown between a formed intermetallic compound layer andthe copper stud. The gap results in a poor electrical and mechanicalconnection between the semiconductor die and the carrier to which it isattached. Without being bound by theory, the inventors believe that thevoid formation at the copper/intermetallic compound interface is causedby either copper oxidation and/or the differences in diffusion rates ofCu, Sn, and Sb.

Embodiments of the invention address these and other problems.

SUMMARY OF THE INVENTION

Embodiments of the invention are directed to stud bumped semiconductordies, semiconductor die packages formed using the bumped semiconductordies, and methods for making the same.

One embodiment of the invention is directed to a method comprising: (a)forming a ball at the tip of a coated wire passing through a hole in acapillary, wherein the coated wire comprises a core and an outeroxidation-resistant coating; (b) pressing the ball to a conductiveregion on a semiconductor die; and (c) cutting the coated wire, therebyleaving a conductive stud bump on the conductive region, wherein theconductive stud bump includes an inner conductive portion and an outeroxidation-resistant layer.

Another embodiment of the invention is directed to a bumpedsemiconductor die comprising: (a) a semiconductor die including aconductive region; and (b) a conductive stud bump on the conductiveregion, wherein the conductive stud bump includes an inner conductiveportion and an outer oxidation-resistant layer.

Another embodiment of the invention is directed to a semiconductor diepackage comprising: (a) a bumped semiconductor die comprising (i) asemiconductor die including a first conductive region, and (ii) aconductive stud bump on the conductive region, wherein the conductivestud bump includes an inner conductive portion and an outeroxidation-resistant layer, and (b) a circuit substrate including asecond conductive region, and solder on the second conductive region,wherein the bumped semiconductor die is mounted on the circuit substrateand the conductive stud bump contacts the solder on the secondconductive region.

These and other embodiments of the invention are described in furtherdetail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) shows a cross-sectional photograph of a copper stud, solderpaste, and, an intermetallic compound layer between the copper stud andsolder paste.

FIG. 1( b) shows a gap that forms between the intermetallic layer andthe copper stud.

FIG. 2 shows a cross-section of a semiconductor die with conductive studbumps on it.

FIG. 3 shows a cross-section of a semiconductor die with conductive studbumps mounted on a circuit substrate.

FIGS. 4-7 show cross-sections of a coated wire, capillary, andsemiconductor die during a process of forming a conductive stud bump.

FIG. 8 is a graph showing ball shear/pull test strength over time for aball that is bonded to a conductive region of a semiconductor die usinga coated wire including a copper core and an outer coating includingpalladium.

The sizes of the various elements in the Figures may not be to scale forease of illustration. Also, in the Figures, like numerals designate likeelements.

DETAILED DESCRIPTION

FIG. 2 shows a stud bumped semiconductor die 100 according to anembodiment of the invention. The stud bumped semiconductor die 100 has asemiconductor die 1 including first conductive regions 2, and conductivestud bumps 27 on the first conductive regions 2.

The semiconductor die 100 may comprise any suitable material (e.g.,silicon, gallium arsenide) and may include any suitable active orpassive semiconductor device. For example, the semiconductor die maycomprise a metal oxide field effect transistor (MOSFET) such as a powerMOSFET. The MOSFET may have a planar or trenched gate. Trenched gatesare preferred. Transistor cells containing trenched gates are narrowerthan planar gates. In addition, the MOSFET may be a vertical MOSFET. Ina vertical MOSFET, the source region and the drain region are atopposite sides of the semiconductor die so that current in thetransistor flows vertically through the semiconductor die.

Each conductive region 2 may be, for example, a conductive land. Eachconductive region 2 may also comprise one or more layers of material.For example, a conductive region according to an embodiment of theinvention may comprise a layer of aluminum with one or more underbumpmetallurgy layers including Ti, Ni, Cr, etc.

Each conductive stud bump 27 may include an inner conductive portion 9and an outer oxidation-resistant layer 8. Each conductive stud bump 27also includes a head 27(a) and a base 27(b). As shown, the head 27(a) isnarrower than the base 27(b). If the semiconductor die 1 comprises apower MOSFET such as a vertical power MOSFET, the conductive stud bumps27 can be connected to the source and gate regions of the MOSFET.

The inner conductive portion 9 preferably comprises copper (e.g., purecopper or a copper alloy). The outer oxidation-resistant layer 8comprises a material that resists oxidation. Suitable materials includenoble metals (and alloys thereof) such as Pd, Pt, Au, Ag, etc. Althoughthe illustrated embodiment shows a conductive stud bump 27 with twodistinct regions (i.e., the inner conductive portion 9 and theoxidation-resistant layer 8), it is understood that the conductive studbumps according to embodiments of the invention may have any suitablenumber of distinct regions.

The oxidation-resistant layer 8 can have any suitable thickness. Forexample, it may have a thickness of about 0.01 to about 0.5 microns insome embodiments. The oxidation-resistant layer 8 can coat at least oneof the sides, top, and bottom of the inner conductive portion 9, and maybe the outermost layer in the conductive stud bump 27.

FIG. 3 shows how the stud bumped semiconductor die 100 can be mounted toa circuit substrate 200. The circuit substrate 200 may include a numberof second conductive regions 30 on a base substrate 15. The basesubstrate 15 may include one or more dielectric layers (e.g., ceramic orpolymeric dielectric layers), and could also include conductive layersbetween adjacent dielectric layers. The circuit substrate 200 could be acarrier for a semiconductor die, a circuit board, or any otherelectrical device for supporting a semiconductor die. For example, thecircuit substrate 200 could alternatively be a leadframe with leads.After mounting the bumped semiconductor die 100 to the leadframe, aninner portion of the leadframe and the bumped semiconductor die 100 canbe encapsulated in a molding compound.

As shown, the second conductive regions 30 may have solder deposits 28on them. Each solder deposit 28 may comprise, for example, solderincluding a material such Pb, Sn, and optionally, Sb. In otherembodiments, the solder could be a lead-free solder such as a soldercomprising Sn, Ag, and Sb. The solder deposits 28 may be on the secondconductive regions 30 before the stud bumped semiconductor die 100 isflipped over and is mounted on the circuit substrate 200. The solderdeposits 28 can be deposited using any suitable process including ascreening process, a pick and place process, or electroplating processor solder dispensing process.

As shown in FIG. 3, the stud bumped semiconductor die 100 may be flippedover and then the heads 27(a) of the conductive stud bumps 27 canpenetrate the solder deposits 28 and may thereafter electrically connectthe first conductive regions 2 and the second conductive regions 30. Asolder reflow process may be performed after mounting. Reflow processesare well known in the art and need not be described in further detailhere. This provides electrical connections between the semiconductor die1 and the circuit substrate 200.

The oxidation-resistant layer 8 of the conductive stud bump 27 protectsthe inner conductive portion 9 from oxidizing. This reduces thelikelihood of forming voids between the inner conductive portions 9 ofthe conductive stud bumps 27, and the solder deposits 28. As explainedabove, the formation of voids is believed to be caused by copper oxideand/or differences in the diffusion rates of atoms such as Cu, Sn, andSb (e.g., for a pure copper stud that contacts a Pb-Sb-Sn solder).

Some embodiments of the invention are directed to methods for formingthe above-described stud bumped semiconductor die 100. A suitable methodincludes forming a ball at the tip of a coated wire passing through ahole in a capillary. The coated wire comprises a core and an outeroxidation-resistant coating. The ball is then pressed to a conductiveregion on a semiconductor die using ultrasonic energy and/or heat tobond the ball to the conductive region. Then, the coated wire is cut,thereby leaving a conductive stud bump on the conductive region. Theconductive stud bump may then be optionally leveled to make its heightconsistent with other conductive stud bumps.

As described above, the resulting conductive stud bump includes an innerconductive portion and an outer oxidation-resistant layer. Then, asdescribed above, the bumped semiconductor die is mounted on a circuitsubstrate. The circuit substrate includes a conductive region and solderon the conductive region, wherein the solder on the conductive regioncontacts the conductive stud bump.

A method for forming a conductive stud bump on a semiconductor die canbe described with reference to FIGS. 4-7. FIG. 4 shows a composite ball32 that forms at the leading end of a coated wire 21. The coated wire 21is fed through a capillary 22 and the composite ball 32 forms at the endof the capillary 22. Thermal energy and/or sonic energy is applied tothe end of the coated wire 21 by using, for example, a gas flame,electric pulses, ultrasonic energy, or the like.

The coated wire 21 and the composite ball 32 each comprise a conductivecore 19 and an outer oxidation-resistant coating 18. The coated wire 21may have a diameter less than about 1 mm. The thickness of theoxidation-resistant coating 18 may be on the order of about 0.01 toabout 0.5 microns in some embodiments. The conductive core 19 maycomprise copper (or a copper alloy) and the oxidation-resistant coating18 may comprise a noble metal such as Pd (or alloy thereof).

The capillary 22 (with ultrasonic energy and/or heat) can be used tocompress the composite ball 32 to the first conductive region 2 of thesemiconductor die 1. By using a thermo-compression bonding process,and/or by using ultrasonic energy, the composite ball 32 is secured tothe first conductive region 2. The pressure applied to the compositeball 32 may vary, and could be, for example, about 20 to about 45 gramsper composite ball 32. The ultrasonic energy that can be applied may bein the range of 60 kHz.

Thereafter, the capillary 22 moves along a path 25 forming a loop shownin FIG. 6 and, as the capillary 22 moves downward as shown in FIG. 7,the coated wire 21 is cut with the edge 110 of the capillary 22. Aninert or reducing gas such as nitrogen or N₂H₂ can be supplied to theconductive stud bump as it is being formed to reduce the likelihood offorming oxides.

Next, the resulting conductive stud bump can be subjected to leveling byapplying pressure (with a flat surface) to the upper surface of theconductive stud bump. By applying pressure (e.g., of about 50 g/bump) tothe top of a recently formed conductive stud bump, it is possible tomake the height of the bump consistent with other bumps.

An apparatus for performing the above process is commercially availablefrom ASM, Inc. of Singapore (e.g., ASM AB339). A KNS 1488L turboconventional wire bonder, commercially available from Kulicke & Soffa ofWillow Grove, Pa., could also be used.

Using a coated wire in the stud-bumping process described above has anumber of advantages. First, the copper in the formed conductive studbumps according to embodiments of the invention is protected by theoxidation-resistant layer and the conductive stud bumps have a betterpull strength as compared to stud bumps that have only copper in them.FIG. 8, for example, shows a graph showing ball shear/pull test strengthover time for a ball that is bonded to a conductive region of asemiconductor die using a coated wire including a copper core and anouter coating including palladium. As shown in the graph, the shearstrength and pull strength of a ball that is bonded to a conductiveregion of a semiconductor die are high. Second, the coated wires thatare used in embodiments of the invention have a long shelf-life and along process life-time. The oxidation-resistant coating in a coated wireprotects the core of the wire from becoming oxidized, thereby improvingits shelf-life. Third, as compared to a Cu stud bump formed from acopper wire, a Cu/Pd stud bump formed from a Cu/Pd wire reduces thelikelihood of forming voids at the Cu/intermetallic compound interface.This improves the overall reliability of a package that is formed usingthe coated wire. Fourth, the oxidation-resistant layer in the conductivestud bump prevents oxidation of the inner conductive portion, therebyimproving the reliability of the conductive stud bump over time. Fifth,the oxidation-resistant layer in the conductive stud bump acts as abarrier metal for the formation of intermetallics that are formed due tothe common use of solder materials. Sixth, embodiments of the inventionprovide a robust, solderable interface between the bump and the circuitsubstrate. Seventh, since a substantially solid conductive stud bump isused in embodiments of the invention, less solder can be used ascompared to a process that uses only solder as an interconnect material.Consequently, a full reflow process (as described above) need not beperformed so that the likelihood of forming intermetallic compounds isreduced in embodiments of the invention.

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention inthe use of such terms and expressions of excluding equivalents of thefeatures shown and described, or portions thereof, it being recognizedthat various modifications are possible within the scope of theinvention claimed. Moreover, any one or more features of any embodimentof the invention may be combined with any one or more other features ofany other embodiment of the invention, without departing from the scopeof the invention.

1. A method comprising: (a) forming a ball at the tip of a coated wirepassing through a hole in a capillary, wherein the coated wire comprisesa core and an outer oxidation-resistant coating; (b) pressing the ballto a conductive region on a semiconductor die; and (c) cutting thecoated wire, thereby leaving a conductive stud bump on the conductiveregion, wherein the conductive stud bump includes an inner conductiveportion and an outer oxidation-resistant layer.
 2. The method of claim 1wherein the core comprises copper and the coating comprises a palladium.3. The method of claim 1 wherein the semiconductor die comprises avertical power MOSFET.
 4. The method of claim 1 wherein theoxidation-resistant layer comprises a noble metal.
 5. The method ofclaim 1 wherein the conductive region on the semiconductor die is afirst conductive region, and wherein the method further comprises, after(c): (d) mounting the semiconductor die on a circuit substrate, whereinthe circuit substrate includes a second conductive region and solder onthe conductive region, wherein the solder on the second conductiveregion contacts the conductive stud bump.
 6. The method of claim 5wherein the solder comprises Pb and Sn, or a Pb-free solder. 7-14.(canceled)
 15. The method of claim 1 wherein the oxidation-resistantlayer comprises a thickness of between about 0.01 to about 0.05 microns.16. The method of claim 15 wherein the semiconductor die comprises avertical power MOSFET.
 17. The method of claim 15 wherein thesemiconductor die comprises a vertical power MOSFET comprising atrenched gate.
 18. The method of claim 15, wherein in step (b), thepressure applied is between about 20 to about 45 grams.
 19. The methodof claim 18, wherein in step (b), ultrasonic energy is applied to theball while applying pressure.
 20. The method of claim 18 wherein theconductive core comprises copper.